Circular coaxial line is a well known type of transmission line suitable for signals at RF frequencies. Another type of well known type of transmission line is the coplanar waveguide (CPW) transmission line. In some applications, it is necessary to provide a transition between these two types of transmission lines.
The Handbook of Microwave Integrated Circuits, R. Hoffman, 1987, Artech House, pg. 88, describes a conventional coaxial-line-to-microstrip connector technique in which the circular coaxial line interfaces directly into coplanar waveguide (CPW). The performance of this connection is not optimum because the E field distribution of the CPW is concentrated along a line as opposed to radially across a plane. FIG. 1A shows the E field configuration of a conventional coaxial line. FIG. 1C shows the E field configuration of a coplanar waveguide. Any discontinuous field distribution in this conventional connector will result in degraded RF performance in terms of poor match and increased losses due to the generation of radiation and higher order waveguide modes.